Figure 1.

The linear model of progressive differentiation. (A) The differentiation of a naïve T cells (TN) cell into an activated phenotype that ultimately results in cell death. Once a CD4⁺ T cell reaches its full differentiation potential, and depending on stimuli, it can be defined as one of seven major effector helper subsets, with distinct surface markers, cytokine secretion profiles and overall functions. (B) As a cell progresses down this linear path, it may diverge at any point depending on the strength and consistency of the stimulus. This divergence results in a memory cell phenotype depending on the early level of differentiation the cell has already experienced. These memory cells are defined by the expression of a range of surface markers and are distinguished by the strength of effector functions and proliferative capacities. According to this model, once a memory cell is reactivated, the cell must only travel down the linear progression to become a short-lived effector T cell (TEFF). This process cannot be reversed to an earlier subset.